Support `working-directory`

[asomers-ax]

Do the checklist before filing an issue:

• Is this related to the actions-rs Actions?
  If you think it's a problem related to Github Actions in general, use GitHub Community forum instead: https://github.community
• You've read the Contributing section about feature requests: https://github.com/actions-rs/.github/blob/master/CONTRIBUTING.md#feature-requests
• Is this something you can debug and fix? Send a pull request! Bug fixes and documentation fixes are welcome.

Motivation

Other github actions support a working-directory attribute that changes the directory before running any command. actions-rs should do that too. For example, trying to do cargo build --feature=XXX in a workspace won't work; the --feature= switch doesn't work in workspaces. Instead, you must cd into an individual crate to build it with specified features.

Workflow example

    - name: test-with-XXX
      uses: actions-rs/cargo@v1
      working-directory: my_subcrate
      with:
        command: test
        args: --features=XXX